As a coating method which forms a coating film of a thin uniform thickness on a surface part of a coatable material, the electrostatic powder coating method is known. As one example of such an electrostatic powder coating method, the method of coating a metal cylindrical member which uses a resin powder as a powder coating such as in PLT 1 is known. In electrostatic powder coating, first, the resin powder is charged by application of static electricity. Next, a coatable object charged with static electricity of the opposite polarity is coated with the charged resin powder to make the powder deposit on the surface of the coatable object. Finally, the coatable object is heated to make the resin powder which is deposited on the coatable object melt and form a coating film on the surface of the coatable object to thereby complete the electrostatic powder coating process. The resin powder will hereinafter be referred to as “powder coating”.
The powder coating art up to now has suspended the coatable material in a coating booth larger than the coatable material to make it the ground potential, mixed the powder coating with a flow of air for transport, charged the powder coating with static electricity at the spray port of a coating gun, and sprayed the powder coating on the suspended coatable material to coat it. For this reason, the volume in which the powder coating scattered was large at the time of coating the powder coating, so the coating booth became large in size. Further, since the coating booth was large in size, the required capacity and size of the powder collector for collecting and recovering the powder coating after coating also became large. As a result, even if the coatable material were small in size, a large sized coating booth and powder collector were required for powder coating. It was difficult to reduce the size and streamline the powder coating facilities.
Furthermore, in powder coating, the ratio of the powder coating which is deposited on the coatable material in a coating booth is about 30%. The remaining powder coating is recovered for reuse, but after several times of use, the powder coating degrades and has to be replaced. The final utilization rate was about 90%. Therefore, it has been desired to raise the rate of deposition of the powder coating on the coatable material in the coating booth and improve the final utilization rate of powder coating.
On the other hand, the inventors noted that among metal cylindrical members which are powder coated, there are ones which do not require coating at their internal circumferential surfaces and discovered that for this type of metal cylindrical member, it is possible to improve the structure of the coating booth of the powder coating system to reduce the size and possible to improve the final utilization rate of the powder coating.